Optical imaging systems, such as the Cassegrain optical system, are well known. Such systems employ generally circular, facing primary and secondary mirrors arranged coaxially about an optical axis, in a rigid mounting. The primary mirror has a diameter significantly larger than the diameter of the secondary mirror and defines a central aperture through which the optical axis extends. Beams of radiation striking the primary mirror are reflected towards the secondary mirror, from where they are reflected back towards a focus of the secondary mirror, lying on the optical axis. Appropriate imaging apparatus may be arranged at the focus.
Optical imaging systems such as described, have to meet very high criteria of accuracy. Imaging systems for military applications must also meet requirements of ruggedness, compactness, reliability and insensitivity to changes in the thermal environment. The accuracy criteria are determined both by the final geometry of the reflective surfaces defined by the mirrors, and the skill by which the mirrors are assembled in their mounting. Such mounting and assembly is exceeding time consuming and thus expensive. While the geometrical accuracy of the reflective surfaces is a function of the machining process by which the mirrors are formed, assembly of the mirrors in their mounting is performed manually, and the overall system accuracy is thus largely dependent on the skill of the person or persons assembling the system.
Conventional system errors are as follows:
Centering between the two mirrors in relation to the optical axis: 0.05 mm PA1 Perpendicularity of the two mirrors to the optical axis: 0.05 mm PA1 Distance between mirror traces: 0.1 mm.
As will be appreciated by persons skilled in the art, while system errors such as listed above may be acceptable for many applications, single axis optical systems having inherent system errors such as those listed are not suitable for imaging applications wherein greater accuracy is needed.
Furthermore, the very high complexity in manually assembling these optical systems so as to maximize their accuracy takes many hours and results in a relatively high cost product.
In an article entitled "Aplanatic two-mirror compact collimator" by I.-Fu Shih et al SPIE Vol. 554, International Lens Design Conference (1985), pp 265-272, there is described a collimator, which is not suitable for imaging but which employs two mirrors diamond turned on a single piece of metal.